The technology for forming nonwoven fabric in the dry way involves forming laps of fibres or of filaments which are subsequently consolidated by the binding of the fibres mechanically (needling), chemically or thermally.
Multi-layer nonwovens are already known in the state of the art.
U.S. Pat. No. 3,298,080 discloses a process for producing a multi-layer needled nonwoven having a fineness gradient. The inner layers have the finest fibres. Moreover, the orientation of the fibres varies from layer to layer. In this technique, the lap is formed by superposing the various loosely entangled fibre layers, and then the needling is carried out in order to bind the various layers to one another. The entanglement of the fibres increases towards the centre of the lap. Consequently, primarily the inner layer is rigid, the outer layers being more flexible.
U.S. Pat. No. 5,106,679 discloses a three-layer self-supporting composite material which can be used as the interior ceiling of a vehicle. Rigidity is obtained by means of a fibrous core comprising palm or flax fibres and a fibrous reinforcing material, this core being contained between two surface covering layers.
In view of the heterogeneousness of its composition, such a product cannot be entirely recyclable.
Document US-A-4,840,832 also discloses a flexible and elastic vehicle ceiling consisting particularly of a nonwoven composed of binding fibres. In this process, polymer fibres are, where appropriate, bound together by means of light needling at a multiplicity of points, so as to form a coherent lap of a stability making it possible to handle it subsequently and to wind it. This nonwoven does not achieve the rigidity desired in order to be self-supporting.
It is also known to obtain needled and thermally bound three-layer nonwovens having high rigidity. In fact, international patent application WO-A-92/05949 discloses a process for producing a rigid nonwoven composed of 3 layers. The outer layers are composed of fibres having a relatively low melting point. The inner layer is composed of a mixture of fibres of the same type with fibres having a higher melting point. In this process, each layer is previously bound and wound, and then the various layers are bound to one another by needling. This needling is therefore carried out through the various layers and, moreover, drives the fibres of the outer layers into the inner layer. Subsequently, the three-layer composite is heated to a level higher than the melting point of the fibres of the first type, so as to form a plasticized structure.
There is no provision for this product thereafter to be moulded, and it is used solely in sheet form.
Patent application EP-A-0,388,062 discloses a process for the production of an absorbent material composed of a nonwoven bound by needling, the needling of which is not carried out through the entire thickness of the fibre lap. This is referred to as progressive needling. A nonwoven of progressive density, useful as absorbent material, is then obtained. This nonwoven consists of a single fibre layer and is not thermo-formable.
Patent application EP-A-0,476,538 discloses a process for the production of a needled nonwoven moulded by thermal bonding and used in the motor-vehicle industry. This product is composed of fibres of polyester, polyamide or polyolefin and comprises two types of fibre, namely adhesive fibres and non-adhesive fibres. The adhesive fibres are highly amorphous fibres, that is to say fibres which soften and crystallize permanently during the thermal binding. Once bound, this nonwoven can therefore no longer be thermoformed by moulding.